1. Field of the Invention
This invention relates to paving compositions comprising a mixture of aggregate and bituminous emulsions. In particular, the present invention relates to the preparation of rapid set high float emulsions or high float emulsions with higher demulsibilities than obtained with common "high float" emulsifiers such as crude tall oil. High float medium set emulsions can be obtained as well. More particularly, this novel process is capable of producing these emulsions utilizing "difficult" asphalts.
2. Description of the Prior Art
Generally, there are two ways to produce such a mixture which, after application onto a worn out road surface, results in a strong matrix suitable to bear heavy traffic. One way to produce an aggregate asphalt emulsion mix is to apply emulsion onto the road surface to be repaired and spread aggregate on top. The stones, which make up the aggregate, sink into the emulsion; and upon setting of the emulsion, which is the result of evaporation of the water from the emulsion, the stones are locked into place; and a strong matrix is obtained. Emulsions used for this type of application are generally referred to as rapid setting emulsions. The American Society for Testing and Materials (ASTM) has promulgated certain testing methods and standards for asphalts and asphalt emulsions. Here, the main specification for anionic rapid setting emulsions according to ASTM-D977 is a minimum demulsibility of 60% when treated with a 0.02N CaCl.sub.2 solution.
The second method to achieve an aggregate-emulsion mix is by mixing the required amount of aggregate and asphalt emulsion at a separate location in a central mixing plant. These mixes are transported to the locations where they are applied. One important feature of the asphalt emulsions to be suitable for this technique is the ability to produce a mix which still can be handled after transport to the location of application. The most expeditious way to do this is to use cutback asphalt from the emulsion. Cutback asphalt is generally referred to as a blend of fuel oil or other petroleum distillates and asphalt. The content of oil may be in the range of 1-15% based on asphalt. The advantage of using cutback asphalt for these mixes is the fact that even when the emulsion in the mix breaks prematurely, the viscosity of the asphalt is still low enough that the mix can be handled. Emulsions used for this type of application are referred to as medium setting emulsions. According to ASTM-D977, they are distinguished from rapid setting emulsions in that no minimum demulsibility is required.
Both cationic and anionic emulsions are applied in the field. The ionic character of the emulsion is distinguished by the type of emulsifier which is used for the preparation and especially by the pH value of the emulsion. Cationic emulsions are generally prepared at pH values between 1.5 and 7.0. Nitrogen containing chemicals, such as fatty amines, fatty diamines, and fatty amidoamines, fatty quaternary-ammonium salts or aminolignins, are used as cationic emulsifiers. Anionic emulsions are prepared at pH values ranging from 7 to 12. Fatty acids or fatty sulfonic acids, which upon treatment with alkali, such as sodium hydroxide or potassium hydroxide, form anionic soaps, are the most effective anionic emulsifiers.
One feature of bituminous pavements regarded as particularly objectionable is the tendency to soften in hot weather resulting in shoving, rutting or bleeding (migration of the binder to the surface) of the matrix. While hard asphalts are not softened at increased temperatures, they are hard to emulsify and show objectionable properties at low temperatures, such as premature cracking.
By employing a suitable emulsifier system for the preparation of the asphalt emulsion, soft asphalts can be modified in a way that after the emulsion has set, the asphalt will be modified so that the temperature range where it possesses decreased flow is extended. Low temperature susceptibility is also improved by maintaining its ductile properties in cold weather (less brittle at low temperature). In general terms, by this modification of the asphalt, the deformation of the road surface in hot weather is prevented, resulting in loss of aggregate from the surface. Emulsifiers, where the hardness at higher temperatures of the asphalt is increased and the hardness at lower temperatures it not adversely affected, are disclosed by K. E. McConnaughay in U.S. Pat. No. 2,855,319.
The most effective emulsifying agent is tall oil soap, obtained as the by-product of the kraft pulping process whereby chipped pinewood is digested with sodium hydroxide and sodium sulfide. If crude tall oil is employed to prepare the emulsion, generally 2% to 4% based on the weight of asphalt is sufficient to obtain high float emulsions.
The compositions of various asphalt emulsions are evaluated by a standard test designated ASTM-D244. An integral part of this residue by distillation test is the exposure of a representative sample of the emulsion to high temperatures (around 260.degree. C.) for up to one hour. The resulting "emulsion residue" is utilized to measure both the increased melting point and the resistance to flow at elevated temperature via the "float test." In this test (ASTM-D139) a plug of "emulsion residue" is solidified in a brass collar held by an aluminum float which is placed into a water bath heated to 140.degree. F. The time required for the water to break through the plug is determined. The asphalt emulsion residue forms the high float test when the collar floats for a least 1,200 seconds. The "residues" of the emulsion typically are considered to be what remains from an asphalt emulsion after the water is evaporated. The significance attached to the high float property is that the residue from a high float emulsion will not flow under the force of gravity at temperatures as high as 140.degree. F. As a result of the high amounts of tall oil soap emulsifier, these high float emulsions show the characteristics of medium setting emulsions since the demulsibility with calciun chloride solution is less than 60% (in most cases less than 20%), which minimum value is specified for rapid setting emulsions (ASTM-D977).
"Rapid setting" emulsions are useful for seal coats, surface treatments, penetration treatment of macadam, etc. Preparation of rapid setting high float emulsions is disclosed by L. F. Ostermeyer et al., in U.S. Pat. No. 4,433,084. In one embodiment of the Ostermeyer et al., process, asphalt cement is mixed with tall oil or tall oil derivatives, such as tall oil pitch or black liquor soap skimmings, at levels of 1-5% based on the weight of the blend prior to emulsification. The blend is then emulsified by conventional techniques to provide a rapid setting emulsion. According to a second embodiment, tall oil pitch or derivatives (such as tall oil heads, tall oil fatty acid, or rosin) are reacted with strong base, such as sodium hydroxide or potassium hydroxide, to form a suitable modifier which is added to the asphalt cement prior to emulsification at a dosage of 0.4-10% based on the weight of the asphalt. In a third embodiment, an alkaline soap of tall oil pitch (or derivative) is formed and added to the asphalt cement prior to emulsification. Rapid setting high float emulsions are also known to be obtained by incorporating 1-5% of a styrene-butadiene-styrene block copolymer or styrene-isoprene-styrene-block copolymer into the asphalt cement prior to emulsification.
In U.S. Pat. No. 4,676,927 to Schilling et al., the use of blends of tall oil or tallow fatty acids and their reaction products with acrylic acid, fumaric acid or maleic anhydride has been disclosed. Vanderzanden in U.S. Pat. No. 4,629,511 discloses the use of combinations of anionic emulsifiers, such as fatty acid sodium salts and fatty acid esters, such as methyl oleate for the preparation of high float rapid set (HFRS) emulsions.
Evaluation of these types of emulsifiers with several asphalts manufactured from a variety of crude oil sources has shown that the prior art emulsifiers described above will not give HFRS emulsions meeting the set specification with certain types of asphalts. For example, U.S. Pat. No. 4,676,927 to Schilling et al., teaches the use of tall oil soaps as anionic emulsifiers to produce high float (HF) emulsions. However, increasing float times by increasing the fatty acid content of an emulsion also results in a corresponding decrease in demulsibility. Therefore, any HF emulsion thus created will show the physical characteristics of a medium setting emulsion (i.e., the demulsibility with the calcium chloride solution will be less than the 60% minimum specified by the ASTM for rapid setting emulsions).
The object of this invention is to provide a novel method to produce asphalt emulsions with all asphalts, including "difficult" asphalts, whereby the emulsions meet HFRS specifications and HFMS specifications. "Difficult" asphalts are those asphalts which are known to be very difficult to emulsify under normal conditions to meet high float rapid set specifications and are characterized primarily by high wax content and by high napthenic acid content, giving the asphalt an acid number of from 3 to 5. Such "difficult" asphalts include, but are not limited to, California Basin and Canadian Crude asphalts.